I was wondering if it is possible to have the Sun acting as an aureole / halo and how to achieve desired effect?
Yes, it is possible. However, without quite a bit of practice (both in setting up and using your camera, and in setting up the shot), I recommend you not try for the shot, and just enjoy the eclipse. You will only have at most 2 1/2 minutes of totality, depending on location.
I recommend not trying to take this shot because of distance and logistics. In order to have the eclipse appear to be a halo around a person's head, the apparent size of the solar ring must be larger than the apparent size of the person's head. That means the person must be far enough away that their head is "smaller than the moon" (in terms of angle of arc).
How far must a person be? First, the sun and moon, both every similar in size in the sky, are about 0.5° of arc. According to Wikipedia, depending on exactly how you measure, the "typical" adult head is roughly 15–21cm across (varies by gender, and whether measure width or front-rear depth, etc.).
Similar to the definition of a parsec, we need to calculate the minimum distance from the camera, x, such that d (15–21cm) subtends an angle of 𝜃 (0.5° of arc). This is given by the forumula,
x = (d/2) / tan(𝜃/2)
By the small angle approximation, tan(𝜃) = 𝜃 for small values of 𝜃, in radians. Therefore, simplifying the expression, in terms of degrees of arc we get:
x = 180∙d / 𝜋∙𝜃
Plugging in 𝜃 = 0.5°, d = 0.21m (worst-case, largest head), we get a minimum distance from the camera of 24 meters.
Okay, placing the subject at least 24 meters from the camera is not a big deal, right? Not so fast. Depending on your location, at max totality, the eclipse will occur
- at roughly 120° azimuth (east-southeast) at 42° elevation, in western Oregon;
- at around 170° azimuth (almost due south) at 62° elevation, in St Joseph, MO;
- at around 228° azimuth (southwest) at 61° elevation, near Charleston, SC.
I've highlighted the angular elevation, because it means that most of the minimal 24 meters distance from the camera to the subject is vertical. Therefore, siting will be the critical component to set up this shot. This means placing the subject at the top of an embankment or small cliff, or on top of a 4–6 story building.
Other factors making this shot difficult include:
communication: Giving precise directions to the subject / model is probably doable by shouting, but because the photographer will be looking directly towards the light source (the eclipse), it will be difficult for him to see the subject being shot. Also, because of the vertical distance separation, it won't be possible to run to the subject to give direction or provide setup assistance during the eclipse totality.
lighting: Even though the environment will be darkened during the eclipse, the main source of light in the scene is still the solar halo. And the camera will be pointed directly at it. You will need to provide some lighting on the subject in order for their face to be visible in the picture. A large reflector might work, but a low-power flash might be required. If you're not prepared to use a radio flash trigger, and understand and know how to set up off-camera flash, this will certainly be too much to handle during the excitement of an eclipse.
(Also, please don't use flash during an eclipse if you're near other people. It's incredibly annoying and distracting, and inconsiderate of their enjoyment of the eclipse.)
distance: Note that I emphasized minimum distance between camera and subject. You probably want to have the solar ring be separated from the subject's head by a visible margin so that the ring isn't "tight" around the subject's head. Thus, you need to add more distance. You should aim for 30-40 meters distance for the largest diameter head, perhaps 25-35 meters if the subject has a small head.